The present invention concerns a dimension-measuring device with magneto-resistive electrodes, and a method for measuring dimensions by means of such a device.
Electronic devices for measuring length or angular position in industry for example must generally satisfy several partially contradictory requirements. They must furnish sufficient precision and resolution and be usable in environments subject to vibrations or to pollution such as dust, oil or humidity. Also expected of such sensors are easy integration into small-size apparatus, without significant adjustment or adaptation, high-speed measurement and low power consumption at the lowest possible cost.
Different types of measuring devices, based upon different physical principles, have been developed to satisfy these various requirements. In particular, measuring systems utilizing capacitance variations caused by the movement of a sensor opposite a scale have been used abundantly in portable devices such as calipers for example. These devices must be kept clean enough to function and are thus poorly adapted to operating in an environment that is humid or subject to the spraying of lubricant or cutting oil for example. Devices for measuring length based upon the principle of magneto-resistive electrodes have been proposed, for example in patent document DE4233331 (IMO), that offer much greater resistance to soiling. The device described in this document comprises a sensor provided with a network of magneto-resistive electrodes connected so as to define two Wheatstone bridges. The sensor is mounted on a slide and can be moved opposite a magnetized scale with a magnetization period xcex. Movement of the sensor opposite the scale causes a modification of the magnetic field applied on the various magneto-resistive electrodes of the sensor and thus a change in their resistance. By applying a voltage to the Wheatstone bridges, one gathers at their outputs an electric signal being a periodic function of the position of the sensor along the scale.
The two Wheatstone bridges are constituted of four magneto-resistive electrodes phase-shifted by xcex/2. The corresponding electrodes of each bridge occupy positions phase-shifted by xcex/4. The electrodes of the two measuring bridges are intermixed. The aforementioned document further suggests the use of barberpole structures, which allow the direction of the current vector I to be modified. As the resistance of a magneto-resistive electrode is a function of the angle between the magnetization vector and the current vector, the barberpole structures make it possible to control the direction and amplitude of the variation in resistance of the electrodes caused by the movement of the sensor.
Each branch of the Wheatstone bridge is constituted of a single magneto-resistive electrode whose width must be sufficient to react to the relatively small magnetic fields generated by the scale. The resistance of the branches of the bridge is thus reduced, and substantial currents circulate across the measuring bridges. The power consumption of this device is consequently high.
The patent document U.S. Pat. No. 5,386,642 (Heidenhain) describes a sensor in which the electrodes are organized into measuring bridges, each branch of which is constituted of several magneto-resistive electrodes of the same phase and connected in series. The resistance of the branches of the bridge is thus greater, which allows the power consumption to be appreciably reduced. However, the consumption of this type of sensor remains too high to contemplate its use in electrically autonomous apparatus, for example in portable precision calipers.
The patent document JP-01-212313-A describes an electronic circuit capable of being used in a dimension-measuring device with magneto-resistive electrode, in which the magneto-resistive electrodes are fed with continuous power supplies.
The patent document JP-A-61-173113 describes a new way of connecting the magneto-resistive electrodes in a device for measuring angular dimension, in order to reduce the power consumption. The reduction in consumption is due to a particular connection of the electrodes.
Document EP0924491, the content of which is hereby incorporated by reference, describes an electronic circuit for a dimension-measuring device of the magneto-resistive type, in which the feeding of the measuring bridges is periodically reduced or interrupted, so as to lessen the dissipation of energy in the magneto-resistive electrodes. By feeding the measuring bridges with a cycle ratio of 1/n, it is thus possible to divide by n the electric consumption in the electrodes.
This circuit also describes a standby-mode circuit enabling the electric consumption to be reduced when the device is not used. In stand-by mode, the display and circuit controller are switched off, the reading of the measurement is thus impossible. These components are then reactivated when a movement of the sensor is detected by means of the magneto-resistive electrodes that must remain under tension.
The standby mode described in EP0924491 is thus not very advantageous, since the device in standby is unusable for displaying measurements but nevertheless continues to use considerably current for feeding the magneto-resistive electrodes.
An aim of this invention is to realize a measuring device with magneto-resistive electrodes different from the prior-art devices and having an electric consumption that is equal or lower.
In particular, an aim of the present invention is to realize a portable measuring such as a battery-powered slide caliper.
According to the invention, these aims are achieved by means of a circuit having the elements of the independent claim, alternative embodiments being furthermore indicated in the dependent claims.